Prosthesis part

ABSTRACT

A prosthesis part can be implanted without the use of cement. The prosthesis part comprises: a cage structure, which unites with the spongy substance of the bone; a carrier plate which is connected to the cage structure; and a carrier post for a joint part. The carrier post is supported by the carrier plate. The carrier plate has openings through which a severing instrument, such as a chisel or milling cutter, is introducable in order to separate the cage structure at least to a large extent from the cortical substance of the bone.

The invention relates to a prosthetic part to be implanted into the endof a thighbone adjacent to the hip joint.

A prosthetic part of this type is described in DE 37 07 518 A1.

After such a prosthetic part has been implanted, spongy substance growsbetween the anchorage pillars of the cage structure, as a result ofwhich the anchorage section of the prosthetic part is dynamically andfirmly connected to the end of the thighbone. After they have healedinto the bone, prosthetic parts of this type which can be implantedwithout the use of cement are so firmly connected to the bone thatremoval which may possibly become necessary often necessitates openingof the sections of the cortical substance of the bone adjacent to theanchorage section.

By means of the present invention a prosthetic part is to be developedfurther in such a way that removal of the prosthetic part from thethighbone is possible without removing parts of the cortical substance.

In accordance with the invention this object is achieved by means of aprosthetic part having the features specified in claim 1 or 6.

In the case of the prosthetic part according to the invention, passagesare present or can easily be created by removing webs of materiallocated beneath indentations or by drilling the supporting plate free,through which passages a chisel or a slender milling cutter can beguided. In this way a slit can be produced in the section of the spongysubstance adjacent to the cortical substance, as a result of which thecohesion between the spongy substance that has grown into the cagestructure and the cortical substance is weakened. The bridges of spongysubstance leading to the cortical substance that cannot be removed bythe chisel or by the milling cutter can then be broken up by mechanicalloading of the prosthetic part (e.g., impact and/or twisting, withapplication of considerable force). After this, the prosthetic part canbe taken out of the bone.

Advantageous further developments of the invention are specified in thesubordinate claims.

In the case of a prosthetic part the volume of spongy substance that hasgrown in is separable from the cortical substance of the bone on the twoprincipal surfaces.

The further development of the invention permits an incision to beproduced in the volume of spongy substance which may extend practicallyover the entire surface of contact with the cortical substance. In thisway it is possible for the prosthetic part to be removed in astraightforward manner.

The further development of the invention is advantageous with regard toparticularly rapid opening of the depression, whereby a secure andload-bearing connection of the edge of the plate section to the interiorof the plate section is guaranteed by virtue of the depressions thathave only slight depth. In the depressions having only slight depth theweb of material can be removed by means of a milling cutter, whereby thewalls of the depression may serve as a guide for the milling cutter.

The further development of the invention permits the setting angle ofthe chisel or milling cutter which is introduced into an opening or intoan opened depression to be varied.

In the case of a prosthetic part a drill for drilling free the points ofattachment of the anchorage pillars pertaining to the cage structure canbe simply and precisely aligned with the axis of an anchorage pillar.

The invention is elucidated in more detail below on the basis ofembodiments with reference to the drawing. Shown in the latter are:

FIG. 1 is a lateral view of a prosthetic part for an artificial hipjoint;

FIG. 2 is a view of the right side of the prosthetic part shown in FIG.1;

FIG. 3 is a transverse section through a depression which is provided ina near-edge region of a plate section of a modified prosthetic part;

FIG. 4 is a longitudinal section through the depression provided in anear-edge region of a plate section of the modified prosthetic partaccording to FIG. 3;

FIG. 5 is a similar view to FIG. 4 but wherein a depression havinglittle depth is reproduced;

FIG. 6 is a top view of the plate section of a further modifiedprosthetic part which is provided in the marginal region of the platesection with a succession of depressions of differing depths; and

FIG. 7 is a view similar to FIG. 1, wherein a further modifiedprosthetic part is shown.

Designated overall by 10 in FIG. 1 is a prosthetic part which serves forimplantation into the upper end of a thighbone 12 indicated by a dashedline. To this end the upper section of the thighbone 12 which bears thecondyle is resected and a recess 14 corresponding to the clear outercontour of the prosthetic part 10 is produced in the thighbone.

The prosthetic part has a supporting plate 16 which comprises ahorizontal plate section 18 and a plate section 20 ascending obliquelyupwards to the trochanter major. Moulded onto the plate section 18 is atrunnion 22 ascending obliquely upwards, onto which a condyle 24indicated by a dashed line is capable of being mounted.

Cast onto the underside of the supporting plate 16 is a cage structurewhich is designated overall by 26. This cage structure consists of fourcurved longitudinal anchorage pillars 28, 30, 32, 34 which substantiallypredetermine the shape of a curved pyramid having a rectangulartransverse cross-section. From the upper end of the plate section 18there emanate two curved additional anchorage pillars 36, 38, theconfiguration of which is adapted to the trochanter major of thethighbone (femur) and which have a smaller spacing from one another thanthe adjacent anchorage pillars 32 and 34.

In the interior of the clear contour of the cage structure 26predetermined by the anchorage pillars 28-38 additional anchoragestiffening pillars are provided which extend in oblique directionsbetween the anchorage pillars 28-38. Three such stiffening pillars 40,42, 44 are shown.

The anchorage pillars and the stiffening pillars each have a cylindricalcore 46 with axially spaced anchorage collars 48.

The cores 46 of the anchorage pillars typically have a diameter of2.0-4.5 mm, the diameters of the anchorage collars are each about0.8-1.5 mm larger than the diameter of the cores. The axial extent ofthe anchorage collars amounts to about 0.6-1.2 mm, their spacing fromone another amounts to about 3-6 mm.

For the stiffening pillars the cores may have a diameter of about 1.5-3mm, the diameters of the anchorage collars may be about 0.6-1.2 mmlarger than the core diameter, and the axial dimension of the anchoragecollars may amount to about 0.5-1 mm, their axial spacing to about 2-4mm.

Cast onto the lower end of the cage structure 26 is a shaft section 50.The latter has an olive-shaped end section 52. A drill hole 54 extendsthrough the interior of the shaft section 50.

With a view to implanting the prosthetic part described above, the endof the thighbone is resected as described above, the upper edge of thecortical substance being given a contour corresponding to the marginalcontour of the supporting plate 16. The bone material removed from theend of the bone in the course of preparation of the recess 14 is groundin a bone mill and poured into the cage structure 26. The outer surfaceof the shaft section 50 is coated with a cement. The prosthetic part 10is then inserted into the end of the thighbone 12, so that thesupporting plate 16 now forms a new upper termination for the bone. Theprosthetic part 10 is then temporarily fixed with wire.

After the cement has hardened, the prosthetic part 10 is connected tothe upper end of the thighbone in partially loadable manner. In thefurther healing process the ground spongy substance coalesces and fillsout the cage structure, at the same time growing up against the corticalsubstance and residues of spongy substance that have remained on thecortical substance.

Alternatively, the shaft section 50 may not be coated with cement, apossibility which is advantageous with regard to easier later removal ofthe prosthetic part.

After the healing process has been concluded an adhesive connectionconsequently exists between the outer surface of the shaft section 50and the cortical substance of the thighbone, whereas in the region ofthe cage structure 26 there is an uninterrupted volume of spongysubstance which has grown around the cage structure 26.

After the spongy substance has grown into the cage structure 26 a veryfirm and loadable connection exists between the thighbone and theprosthetic part. If for some reason the prosthetic part should have tobe removed from the thighbone at any time, this would probablynecessitate removing a part of the cortical substance surrounding thecage structure 26 and chiseling the cage structure 26 free again with achisel through the window that has been created in this way. This wouldconsiderably restrict the options for a replacement prosthetic part, inparticular it would rule out replacement by means of a prosthetic partthat has to be cemented in.

In order to enable the cage structure 26 to be chiseled free withoutproducing a window in the spongy substance, in the prosthetic part shownin FIGS. 1 and 2 elongated openings 60, 62 are provided in the vicinityof the longitudinal edges of the supporting plate 16. A thin, slenderchisel can be guided through said openings after the prosthetic part 10has healed in, in order in each case to chisel free one of the twoprincipal surfaces of the cage structure 26. After such an operation tochisel the cage structure 26 free, bridges of spongy substance remain,via which the cage structure remains connected to the cortical substancein the vicinity of the upper corners of the supporting plate 16 and onthe narrow sides of the cage structure. These remaining bridges can bebroken open by rotating the prosthetic part 10 about the axis of therecess 14 and/or by inflicting severe blows on the supporting plate 16.

If it is desired that the supporting plate 16 normally seals the upperend of the thighbone 12 completely but that access to the two principalsurfaces of the cage structure 26 should nevertheless be possible,instead of openings it is possible, according to FIGS. 3 and 4, to workwith indentations 64 in the supporting plate 16 which, seen in top view,have the same geometry as the openings 60, 62 shown in FIGS. 1 and 2.However, the indentations 64 have a thin bottom wall 66, which has thedesired barrier effect, but on the other hand they can easily bemechanically destroyed by the chisel which serves to chisel the cagestructure 26 free.

As is evident from FIG. 3, the lateral walls of the indentations 64 orof the openings that are later created from said indentations or ofopenings that are present from the very beginning are convex, asrepresented at 68. This permits the chisel or milling cutter which isused for chiseling the cage structure 26 free to be inclined also in theperpendicular direction relative to the longitudinal direction of theindentation or opening.

In a further modification, according to FIG. 5 use may be made of suchindentations in which the bottom wall 66 makes up an appreciable part ofthe thickness of the supporting plate 16, so that the indentations 64 donot have a negative influence on the mechanical strength of the marginalregion of the supporting plate 16. This design of the indentations 64permits several such indentations to be distributed along the edge ofthe supporting plate 16, as a result of which a larger part of theinterface between cage structure 26 and cortical substance of thethighbone 12 is made accessible. With such a design of the indentations64 removal of the bottom wall 66 is effected by means of a millingcutter of suitable width, said milling cutter being guided by the wallsof the indentation 64 parallel to the edge of the supporting plate 16.

In a further modification of the invention, according to FIG. 6 anarrangement of indentations 64 a and 64 b may be provided on the upperside of the supporting plate 16 continuously following the edge, thebottom walls 66 a and 66 b of said indentations being thin and thick,respectively. The bottom wall of the entire indentation arrangement canthen be opened rapidly by means of a milling cutter which once again ispositively guided parallel to the edge of the supporting plate 16 by thewalls of the indentations, but under usage conditions a very loadableconnection formed by the bottom walls 66 b exists between the edge ofthe supporting plate 16 and the principal section thereof.

In the case of the prosthetic part 10 according to FIG. 7 all thebase-points of anchorage pillars which emanate from the supporting plate16 are marked by a conical marker 70 on the upper side of the supportingplate 16. These markers serve at the same time as centring holes for thepoint of a drill with which the material pertaining to the supportingplate located above these anchorage pillars can be drilled away. Oncethis has been done for all the anchorage pillars the supporting plate 16comes free from the cage structure (thin webs of material possibly leftstanding can be broken in the course of drilling by progress of thedrill), so that said cage structure can be simply chiseled or milled outof the recess 14.

What is claimed is:
 1. A prosthetic part comprising an anchorage section(26, 50) that is introducable into a bone recess (14), said anchoragesection comprising at least partially a cage structure (26), a platesection (16) which extends in a lateral direction beyond the anchoragesection (26, 50), and a supporting section (22) for a joint part (24),wherein the plate section (16) is provided on its upper side withmarkers (70) which are each aligned with a supporting-plateattachment-point of an anchorage pillar (28 to 34) pertaining to thecage structure(26).
 2. The prosthetic part as set forth in claim 1,wherein the markers comprise notches (70) which center a drill-point. 3.A prosthetic part comprising an anchorage section (26, 50) that isintroducible into a bone recess (14), said anchorage section comprisingat least partially a cage structure (26), a plate section (16) whichextends in a lateral direction beyond the anchorage section (26, 50),and a supporting section (22) for a joint part (24), wherein the platesection (16) is provided with at least one opening (60, 62) or at leastone indentation (64) in the vicinity of the edges of the plate section,through which a severing instrument is introducible to cut a connectionbetween the anchoring section and corticalis substance of a bone,wherein the plate section (16) comprises a marginal region having acircumferential arrangement of indentations (64 a, 64 b), and, whereinthe circumferential arrangement of indentations (64 a, 64 b) comprises asuccession of indentations (64 a, 64 b) of differing depths.
 4. Theprosthetic part as set forth in claim 3, wherein the at least oneopening (60, 62) or the at least one indentation (64) is convex.
 5. Theprosthetic part as set forth in claim 3, wherein the openings (60, 62)or the indentations (64) are convex.
 6. The prosthetic part as set forthin claim 3, wherein the circumferential arrangement of indentations (64a, 64 b) comprises a succession of indentations (64 a, 64 b) ofdiffering depths.